The present invention relates generally to an exposure apparatus, and more particularly to an illumination optical system for uniformly illuminating a reticle (or a mask) which has a pattern, in an exposure apparatus used in a photolithography process for fabricating semiconductor devices, liquid crystal display devices, image pick-up devices (CCD, and the like), thin-film magnetic heads, and the like.
Recently, a manufacture of finer semiconductor devices has increased demands for high throughput with a design rule for a mass production line of 130 nm. The fine processing with improved resolution requires the uniform light intensity for illuminating a reticle or a mask and the uniform effective light-source distribution as an angular distribution of the exposure light for illuminating the reticle (or mask) as well as a shortened wavelength of exposure light and a higher numerical aperture (“NA”) of a projection lens.
Shortening the wavelength of exposure light can cause an increased absorption in an optical material, such as a glass material and optical coating, lowering the transmittance disadvantageously. Therefore, instead of a conventional dioptric projection optical system that includes only lenses, use of catoptric (i.e., full mirror type) and catadioptric (i.e., lens and mirror hybrid type) projection optical systems have been conventionally proposed. See, for example, Japanese Patent Applications Publication Nos. 62-115718 and 62-115719.
A projection optical system that uses a mirror usually shields the light near the optical axis, and the aberrational correction addresses only the off-axis image points. As a result, the exposure apparatus transfers a pattern by illuminating an off-axis imaging area. This imaging area is often rotationally symmetrical around the optical axis, and typically has an arc shape with a certain width. The arc shape imaging area can achieve high-throughput if applied to an optical lithography that exposes a large-scale liquid crystal substrate with the mirror optical system.
To uniformly illuminate a mask and to make an effective light-source distribution uniform, a conventional method proposes to combine an illumination optical system with a collimator lens and a fly-eye lens that includes plural fine lenses or lens elements. The fly-eye lens forms a secondary light source corresponding to the number of lens elements near the exit surface, and uniformly illuminate an illuminated surface through superimpositions of beams from plural directions.
Increasing the number of lens element of the fly-eye lens is effective to form a more uniformly illumination area. However, the lens elements are an independent lens respectively, so the cost increases by gaining the numbers of the lens elements. Moreover, it is not easy working to make the fly-eye lens that arranged the lens elements by the common difference of corresponding.
A method of making a diffraction element and a micro-lens element on one glass substrate by a means such as etching and mechanical polishing is researched. See, for example, Japanese Patent Application Publication No. 7-306304. A one that micro element lens with the lens function is directly formed to one glass substrate by using photolithography is called a micro-lens array. The unitary-type (micro) fly-eye lens formed to substrate as one body makes minute 1 mm or less an individual lens element. As a result, the number of irradiation points as an optical integrator increases, and more uniformly effective light-source can be provided to the projection optical system. In addition, a manufacture becomes easy, and low-cost can be achieved.
However, the unitary-type fly-eye lens has a problem in that the uniform illumination distribution cannot necessarily be formed. Thereby, the unitary-type fly-eye lens generates a step with little lens function at interface to form plural lens elements on one substrate three-dimensionally and periodically. This step does not contribute to uniform mask illumination, and the light that passes through the step goes straight.
This light then condenses in a mask plane or an intermediate imaging plane conjugate with it by a function of a subsequent condenser lens. This is because the Koehler illumination substitutes an angular relationship at an incident side of the condenser lens for a positional relationship on an illuminated surface, and the light that goes straight without receiving the lens function in the step of the unitary-type fly-eye lens condenses at one point of the illuminated surface regardless of incident positions upon the condenser lens. The condensing point forms an abnormal point, which is called a hot spot, causes non-uniform light intensity. In other words, the hot spot makes the light intensity distribution non-uniform on the illuminated surface. This results in an abnormal integral exposure dose in the exposure plane and an abnormal critical dimension in the circuit pattern, decreasing the yield.
The unitary-type reflection optical integrator that directly forms fine reflective elements on the substrate has a similar problem.
Accordingly, it is demanded to provide an illumination optical system and an exposure apparatus that can uniformly illuminate an illuminated surface even when using the unitary-type optical integrator.